1. Field of the Invention
The invention relates to a method for thinning a wafer. More particularly, the invention relates to a method for thinning a wafer, which has had certain chip circuits fabricated therein through processes such as patterning, into a final product thickness, before the wafer is diced into chips.
2. Description of the Related Art
For the thinning of a wafer, after the fabrication of chip circuits is finished, a back side grinder is usually used. The back side grinder is a grinding wheel made of abrasive grain, such as of diamond. A wafer is machined to have a predetermined thickness by the rotation of the grinding wheel (which is called back side grinding).
Back side grinding of a wafer is conventionally carried out as follows. As shown in FIG. 2, a wafer 104 having a protective tape 106 of a pressure sensitive type attached to a side (front side), on which chip circuits have been fabricated, is placed on a wafer table 102 having a porous sucking section 102b inside a frame 102a, in such a manner that it is sucked by the sucking section 102b through the protective tape 106. The wafer has a peripheral area having a thickness which is generally smaller than that of the other portion due to edge treatment. The peripheral area does not adhere to the protective tape 106, as shown in the drawing.
A rotational grinding wheel 108 is applied to the back side of the wafer 104 (the upper side when the wafer is sucked to the wafer table 102 with the chip circuits-formed face down, as shown in FIG. 2), for rough grinding. For 6-inch (about 15 centimeters) and 8-inch (about 20 centimeters) silicon wafers, rough grinding conditions are: 3 μm/sec wheel feed rate, 2400 rpm wheel rotational speed, and 200 rpm table rotational speed, for example. Subsequently, finish grinding is carried out at the conditions of, for example, 1 μm/sec wheel feed rate, 2400 rpm wheel rotational speed, and 200 rpm table rotational speed, which is additionally followed by polishing. Polishing conditions are: 150 g/cm2 (14.7 kPa) pressure, 280 rpm buffing wheel rotational sped, and 240 rpm table rotational speed, for example. The wafer 104 is then removed from the wafer table, and the protective tape 106 is peeled off.
In such a thinning process, a wafer to be thinned having a smaller thickness is warped at its periphery and becomes easily cracked during the grinding, although these problems are not occur when the wafer has a greater thickness since the wafer itself is tough. The problems cause a reduction in productivity of final products, and are not preferred.
JP 2001-93867 A describes a method for grinding a wafer and a protective member at the outer periphery of the wafer, as a relevant technique, and discloses a method for grinding a wafer using a protective member, which is removably fitted to the outer periphery of the wafer for the protection of the outer periphery. The protective member has a protective layer and an adhesive layer, and is adhered to the outer periphery (the outer circumference and its proximity) of the wafer and is peelable in the diametral direction.
JP 2001-189292 A describes, as a method for preventing the thickness of the outer periphery of a wafer after grinding from becoming small (“edge rounding”), a method in which an adhesive is applied to a wafer to be attached to a holding table in such a manner that the applied adhesive has a thickness at the outer periphery of the wafer which is smaller than the thickness at the inside.
JP 2002-110601 A also describes a method for preventing edge rounding in a wafer after grinding, in which method a wax is applied to a wafer in such a manner that the applied wax has a thickness at the outer periphery which is smaller than the thickness at the inside.
JP 2002-264008 also describes a technique for preventing the edge rounding in a wafer after grinding, according to which the edge rounding in a wafer is prevented by allowing the outer periphery of the wafer to be warped and deformed depending on the magnitude of a grinding pressure when the outer periphery of the wafer is applied with a grinding pressure no smaller than that applied to the inside thereof during the grinding.